The present invention relates generally to the field of electronics and, in particular, to automatic level control for an input to an analog to digital converter.
Analog to digital (A/D) converters are used in a wide variety of electronic circuits. For example, A/D converters are used in high frequency digital transmission systems that receive analog input signals. The A/D converter is an electronic circuit that receives the analog input signal and produces a digital output signal. The digital output signal is produced based on samples of the analog input signal taken over time and processed through a plurality of output registers. The A/D converter samples the digital signal based on a clock signal. In high-speed applications for digital transmission systems, these clock signals typically operate at speeds in the tens to hundreds of megahertz range.
A/D converters are designed to process analog signals over a specified range of analog signal values. When the input signal exceeds the specified peak input signal level, the output registers of the A/D converter overflow. This occurs even when the input voltage exceeds the peak input level by a value that would cause the A/D converter output to exceed its maximum level by a single least significant bit. Further increases beyond this point produce spurious output signals (distortions) that are proportional to the degree of overload of the A/D converter""s output registers. The spurious signals can be detrimental to the operation of the digital transmitter. However, if this overload is kept at sufficiently low levels in terms of amplitude and frequency of occurrence, then the resultant distortion can be acceptable in many applications.
To reduce the effect of overflow conditions, some electronic systems are adjusted to operate well below the peak input voltage range of the A/D converter. When initially setting up the system, the input voltage is increased gradually while an overflow register of the A/D converter is monitored. When an overflow condition is reached, the A/D converter provides an output of narrow pulses from the overflow register. The frequency of occurrence of the pulses is typically proportional to the severity of the overflow. Based on the monitored output of the overflow register, the input to the A/D converter is adjusted, e.g., by setting an attenuator, such that the expected maximum input signal will not exceed the peak input range of the A/D converter. In some systems, this adjustment is set at or near 50% of the peak input voltage level. Unfortunately, this reduces the effectiveness of the A/D converter and reduces the signal to noise ratio for the electronic device.
In some circuits, it is desirable to maintain the peak input voltage of the analog signal at or near the maximum value for the range of input signals accepted by the analog to digital converter. Thus, automatic gain control circuits have been used in conjunction with analog to digital converters. Typically, the automatic gain control circuit monitors the input to the analog to digital converter. The automatic gain control circuit further generates a feedback signal based on the monitored input signal. The feedback signal is provided to an amplifier to control the level of the input signal to the analog to digital converter. This feedback signal attempts to keep the peak voltage level of the input signal at or near the full-scale value of the input for the analog to digital converter.
Conventionally, the feedback signal is generated using analog circuitry. For example, such feedback loops typically include one or more of the following analog control blocks: a log amplifier, a summing amplifier, an integrator, and a differentiator. Unfortunately, these analog feedback control loops typically suffer from the so-called xe2x80x9cclipxe2x80x9d effect. This means that when the input signal exceeds the full-scale input for the analog to digital converter, the control circuitry is unable to quickly reach steady-state operation. Some control loops attempt to use digital circuitry. However, these digital control loops typically suffer from an additional problem relating to the quantization of the control word used to adjust the gain of the input signal for the A/D converter. In this situation, when the control loop reaches a steady state, it tends to introduce an oscillation effect on the input signal to the A/D converter.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved automatic gain control circuit for an analog to digital converter input.
The above-mentioned problems with analog to digital converters and other problems are addressed by the present invention and will be understood by reading and studying the following specification. Embodiments of the present invention use automatic level control based on an overflow indicator of an analog to digital converter to maintain a peak level of the input signal for the analog to digital converter substantially near a peak level acceptable by the analog to digital converter.
More particularly, in one embodiment an analog to digital (A/D) conversion circuit is provided. The A/D conversion circuit includes an input adapted to receive an analog signal. The A/D conversion circuit further includes a variable attenuator that is coupled to the input and that has a feedback control input. The A/D conversion circuit further includes a converter that is responsive to the variable attenuator and that converts analog input signals to digital output signals. A feedback loop is also provided. The feedback loop is responsive to an overflow indicator of the converter. The feedback loop is adapted to produce a feedback signal based on the overflow indicator and is adapted to provide the feedback signal to the feedback control input of the variable attenuator so as to maintain the peak level of the input to the converter substantially near an acceptable peak input level for the converter.